This change will make it easier to disable screenshot comparison tests
on a specific platform or have per-platform expectations.
Additionally, it's nice to be able to tell if a ref-test uses a
screenshot as an expectation by looking at the test path.
(cherry picked from commit 715f0330070d9f8ecc88a8e66e4d8c85f795d622;
amended to also move
Tests/LibWeb/Screenshot/canvas-stroke-styles.html and support files)
